The properties of polymers depend upon the properties of the catalyst used in their preparation. In catalysts, control of the shapes, sizes, and the size distribution of the catalyst is important to ensure a good commercial workability. This is particularly important in gas phase and slurry polymerization. For example, in order to produce copolymer granules of 1000 μm in size, a catalyst particle size of about 10 μm to about 50 μm is generally preferred for use in the polymerization. In the copolymerization of olefins, a catalyst with a developed system of pores in its structure is often desired. Finally, a catalyst needs to posses good mechanical properties to resist wear during the polymerization process and to ensure a good bulk density of the polymer product. One important aspect relating to the development of a polymerization catalyst is, therefore, the provision of a process for production of a catalyst which allows control and adjustment of the structures and sizes of the catalyst's particles and particle size distribution, and yet remains a relatively simple process.
However, reported processes utilizing catalysts containing magnesium and titanium often require a long series of synthetic steps. The synthetic step are designed to provide a catalyst with a high magnesium content because higher concentrations of magnesium increase the activity of the catalyst and result in polymers having more desirable properties. By providing the catalyst on a support material, many of the synthetic steps can be simplified or eliminated. Unfortunately, even where the catalyst is impregnated on a support material, the amount of catalyst that can be incorporated is limited by solubility of the magnesium component in the preparative solvent.
For typical magnesium sources, such as magnesium halides, their solubility in polar organic solvents actually decreases from about room temperature to the boiling point of such solvents. The reduced solubility is thought to result from the formation of polymeric magnesium halide-solvent complexes with lower solubility, such as MgCl2(TBF)1.5-2. For example, solutions of ultra-pure magnesium chloride in tetrahydrofuran (THF) form a solid having an approximate composition of MgCl2(TKF)1.5 precipitates upon heating and the maximum concentration of MgCl2 obtainable in such a solution is less than about 0.75 moles MgCl2/liter. At about 60° C., near the boiling point of THF, the solubility is noticeably decreased to less than 0.5 moles/liter. However, when commercial grade magnesium chloride is used, its maximum solubility in THF is lowered, presumably due to impurities such as water, to about 0.6 moles MgCl2/liter. In these cases, the solubility is only about 0.35 moles/liter at 60° C. Such low solubility of magnesium sources limits the amount and distribution of magnesium halide that can be incorporated into a supported catalyst particle.
Generally, lower solubility in the solvent results in lower magnesium halide concentrations in resulting catalyst particles. However, another problem associated with the use of magnesium halides is selective precipitation. Magnesium halides tend to form deposits readily on the outer surfaces of a porous catalyst support during the drying process while the transition metal component remains soluble during drying. Thus, the resulting particle has a fairly uniform transition metal concentration distribution. However, the preferential precipitation of the magnesium halide leads to variations in the magnesium:transition metal ratio throughout the catalyst particle. In some cases the magnesium to transition metal ratio at the outer periphery of the particle may be more than ten times the ratio at the center of the particle.
Thus, new supported catalysts incorporating a relatively higher concentration of magnesium halide within the catalyst particle would be useful. Such higher concentrations should be achieved by a process that does not cause problems in later stages of manufacturing.